According to statistics of 155 gas samples from the southern margin of Junggar basin,the N2-enriched gas is mainly distributed in the neogene Shawan Formation,Palaeogene Ziniquanzi Formation and the shallow layers of the north Tianshan piedmont.Chemical and carbon isotopic compositions of the natural gas indicate both N2 and hydrocarbon gases in the natural gases are derived from the Jurassic coal in the southern margin of Junggar basin and not from atmosphere and mantle-volcanic gas.The N2 content in natural gas is relative to reservoir age and buried depth of gas reservoir,the content of N2 is increasing with reservoir age chances from old to young and buried depth from deep to shallow.The relatively high N2 content is controlled by vertical migration of gas.Natural gas is degraded by microbe in the neogene Shawan Formation,Palaeogene Ziniquanzi Formation and the shallow layers at the north Tianshan piedmont.